Fix -mthumb resetting of target cpu

Passing -mthumb with no explicit CPU on the command line
resulted in target CPU changing from the architecture
default to arm7tdmi. Now it does not.
Make thumb the default instruction set for v7m and v7em
Refactor duplicate functions

getARMCPU and getLLVMArchSuffixForARM existed as very similar functions
in both ToolChain.cpp and Tools.cpp. Create a single implementation of
each in Tools.cpp, eliminate the duplicate and share via Tools.h.

Creates an 'arm' namespace in Tools.h to be used by any ARM-targetting tools.
Simple refactoring to eliminate single-use local variable